1. Field of the Invention
The present invention relates to an exposure apparatus for manufacturing semiconductor devices such as IC or LSI, and more particularly to alignment between a photomask and a semiconductor substrate, and to a photomask having reference marks for said alignment.
2. Description of the Prior Art
In modern semiconductor device manufacture there have widely been employed so-called step-and-repeat apparatus for repeatedly exposing a semiconductor wafer to a circuit pattern of a mask. Particularly an exposure apparatus for transferring the circuit pattern of the mask onto the wafer through a reducing projection lens is highly productive as it can achieve exact pattern superposing despite dimensional change of the wafer in various process steps, since the circuit pattern of the mask is superposed onto each of plural patterns already formed on the wafer. Such exposure apparatus is equipped with a two-dimensionally movable stage for supporting the wafer, and the exposure is repeated by stepping said stage by a determined pitch along a determined coordinate system with respect to the image of the circuit pattern of the mask, whereby the circuit patterns are arranged in a matrix on the wafer.
The coordinate of arrangement of the circuit patterns on the wafer coincides with the coordinate system of the stage during the exposure operation, but, when the wafer is exchanged, there is generated a rotational aberration or deviation between the coordinate of arrangement on said wafer and that of the stage.
Said rotational aberration can be corrected by the rotation of wafer, but there still remains a slight rotational aberration. Said slight rotational aberration between the coordinate of arrangement on the wafer and the coordinate system of the stage is generally called wafer rotation. For achieving more precise superposing, there is already proposed a method for correcting the wafer rotation as disclosed in the U.S. Pat. No. 4,385,838 issued May 31, 1983, and the wafer rotation can be corrected by such known method.
However the slight rotational aberration of the reticle or the photomask becomes significant as the size of the circuit pattern to be transferred onto the wafer becomes larger and the line width reaches an order of 1 .mu.m. Such slight rotational aberration of the reticle not only deteriorates the precision of superposing onto the circuit patterns on the wafer but also results in an unsatisfactory superposing of the second and ensuing layers since the circuit pattern of the first layer transferred onto the wafer is slightly rotated with respect to the coordinate system of arrangement on the wafer, so that it becomes impossible to obtain semiconductor devices of the desired performance at a high productivity.